Process variable transmitters are used in industrial process control environments and couple to the process fluid and provide measurements relative to the process. Process variable transmitters can be configured to monitor one or more process variables associated with fluids in a process plant such as slurries, liquids, vapors and gasses in chemical, pulp, petroleum, gas, pharmaceutical, food and other fluid processing plants. The monitored process variables can be pressure, temperature, flow, level, pH, conductivity, turbidity, density, concentration, chemical composition or other properties of fluids. Process variable transmitter includes one or more sensors that can be either internal to the transmitter or external to the transmitter, depending on the installation needs of the process plant. Process variable transmitters generate one or more transmitter outputs that represent the sensed process variable. Transmitter outputs are configured for transmission over long distances to a controller or indicator via communication buses 242. In typical fluid processing plants, a communication bus 242 can be a 4-20 mA current loop that powers the transmitter, or a fieldbus connection, a HART protocol communication or a fiber optic connection to a controller, a control system or a readout. In transmitters powered by a 2 wire loop, power must be kept low to provide intrinsic safety in explosive atmospheres.
One type of process variable transmitter is known as a pressure transmitter. Typically, a pressure transmitter will be coupled to the process fluid through impulse lines. Pressure transmitter operation can easily deteriorate if one or both of the impulse lines becomes plugged.
Disassembly and inspection of the impulse lines is one method used to detect and correct plugging of lines. Another known method for detecting plugging is to periodically add a “check pulse” to the measurement signal from a pressure transmitter. This check pulse causes a control system connected to the transmitter to disturb the flow. If the pressure transmitter fails to accurately sense the flow disturbance, an alarm signal is generated indicating line plugging. Another known method for detecting plugging is sensing of both static and differential pressures. If there is inadequate correlation between oscillations in the static and differential pressures, then an alarm signal is generated indicating line plugging. Still another known method for detecting line plugging is to sense static pressures and pass them through high pass and low pass filters. Noise signals obtained from the filters are compared to a threshold, and if variance in the noise is less than the threshold, then an alarm signal indicates that the line is blocked.
These known methods use techniques which can increase the complexity and reduce reliability of the devices. Moreover, while these methods can sometimes detect a plugged impulse line, they generally cannot detect when deposits begin to collect within the impulse line, but do not yet plug the impulse line. Thus, operation may continue even though the pressure transmitter's ability to sense pressure has been compromised to some extent. There is thus a need for a better diagnostic technology providing more predictive, less reactive maintenance for reducing cost or improving reliability.